<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        /**
         * concat
         * copyWithin
         */
        const arr1 = [1, 2, 3, 4]
        const arr2 = [5, 6, 7]
        const arr3 = [8]
        const arr4 = arr1.concat(arr2, arr3)
        console.log(arr4); // [1, 2, 3, 4, 5, 6, 7, 8]

        console.log(arr4.copyWithin(3, 5, 7)); // [1, 2, 3, 6, 7, 6, 7, 8]
    </script>
</body>
</html>